1. Technical Field
The present invention relates to amplifiers and more particularly to an amplifier for use with a telephone transmitter.
2. Background Art
Since the advent of telephones employing active gain circuitry many such units have found it expeditious to employ for the voice transmitter an electret microphone. Many different circuits have been devised to provide proper amplification for use with the electret as a transmitter. Most of the amplifier circuits employed have one or more disadvantages such as poor frequency response, effective operation only over a narrow range of supply voltage, a tendency to sing in response to acoustic feedback, susceptibility to radio frequency interference, incorrect impedance or current supervision properties. A number of the circuits developed act as voltage sources so that substantial signal loss occurs when the telephone is employed over a long loop, as opposed to those circuits wherein the amplifier acts as a constant current source. Some of the circuitry employed attempts to utilize only a minimum of transistors such as two or three to provide low cost implementation, but accordingly performance suffers as a result.
Recently a number of circuits have been employed which have been made into integrated circuit packages. They sometimes include some of the same problems as described above plus possible large variations in properties from one lot to the next of the integrated circuit units, resulting in a high yield loss and of course variable performance.
More recently new high quality but low cost telephones have been developed for the business and residential telephone subscriber. Quite frequently these units employ telephone transmitters of low output that require amplification of the voice signals picked up by the telephone transmitter. Often these telephone transmitters are of the electret type with an associated built in amplifier, pre-amplifier or buffer amplifier included in the handset or hand held telephone unit in which the telephone transmitter itself is located. The associated receiver also included in the same telephone handset or unit is usually a standard magnetic receiver and may or may not be driven by an associated amplifier. In such telephones the voice network may be located in the handset or the hand held unit or as an alternative may be located in the associated telephone base. Such network circuitry may be of the traditional coupled coil type or may employ an integrated circuit is so called inductorless designs.
Most of these telephones did not take into the account the hazard of feedback seen under certain sound reflective conditions, where output from the handset receiver may be picked up by the microphone and reinforcing the output of the receiver because of the normal side tone available in the voice network causes "singing." "Singing" is defined as an oscillation that occurs usually in the frequency range of 2,000 to 3,000 Hz and is audible without holding the telephone receiver close to the subscriber's ear. This condition typically occurs only under extreme conditions when the handset is placed downward or in a reflective corner environment forming a type of resonant cavity around the microphone and the receiver.
Most previous designs have been found to have this objectionable "singing" characteristic under certain conditions, which can be transmitted over the telephone line to the other party. Early attempts to solve this problem resulted in a reduction in the transmit gain and loss of low frequencies as well as high frequencies. Obviously, such signal reduction is not desirable particularly if the transmitter is to be used on anything other than in a short loop environment. One method used to stop the singing involved, uses a feedback loop from the output of the telephone transmitter amplifier back to its input. This particular technique resulted in substantial loss in transmit level. Accordingly, it is the object of the present invention to provide a new and useful amplifier circuit for use with electret microphones in telephone instruments, which overcome problems and deficiencies outlined above.